House price declines of up to 20 per cent and an 80 per cent crash in residential sales is the doomsday scenario investment bank Citi says is a real possibility if the residential property market rolls over.

Key points:

Prices in residential developments could fall 20 per cent if the property market cools

Historically sales have fallen 70 per cent from peak-to-trough in downturns

The property market generally takes two years to unravel

Citi's residential property team has looked at the impact of a cooling property market on big developers such as Stockland and Mirvac and the picture is far from pretty.

"When the market cools, it freezes — peak to trough volumes can decline as high as up to -80 per cent," Citi's David Lloyd said.

"One industry contact puts a 90 per cent probability on a 10-to-20 per cent decline in house prices in the next 12 to 24 months," Mr Lloyd and his team wrote in a note to investors.

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